The relation between the maximum effective output power (installed or nominal capacity) of a nuclear reactor and the cooling capacity of its emergency cooling system is essentially determined by the so-called decay heat, i.e. the heat produced by radioactive decay of the fission products after shut-down of the fissioning process. The removal of the decay heat is most critical, particularly for light water reactors (LWR), in the first minutes after shutdown. Immediately after shutdown the decay heat corresponds to about 7% of the operating power of the reactor before shut-down. In case of a pressurized water reactor (PWR), without cooling, the decay heat would cause melting of the metal jackets enclosing the fuel pellets within 50 seconds after shut-down from nominal power. Typically, the heat produced by a LWR in the first 200 seconds after shutdown determines the necessary maximum cooling capacity of the emergency cooling system of the reactor.
The decay heat is essentially comprised by three portions originating from
1. the decay of the fission products, PA1 2. the decay of the elements, like .sup.239 U, .sup.239 Np and further actinides, which are formed by neutron capture by the nuclear fuel, PA1 3. the decay of isotopes produced by neutron capture by the fission products.
The presently valid nuclear standards, as the ANS standard ANSI/ANS-5.1-1979/1985 and the DIN standard DIN 25463 (July 1982), which estimate the decay heat and basing on this prescribe the minimum permitted cooling capacity of the emergency cooling system in relation to the maximum operating power (installed capacity or nominal output power) of the reactor in normal operation or, in other words, prescribe the maximum permitted operating power of a power plant in normal operation with a given cooling capacity of the emergency cooling system, are based on relatively uncertain assumptions. The experimental information available at present is incomplete, as discussed in the report on the "Invited Paper presented at International Conference on Nuclear Power Plant Aging, Availability Factor and Reliability Analysis", San Diego, Calif., 8-12 Jul. 1985, Max-Planck-Institut fur Kernphysik, Heidelberg, F. R. Germany, MPI H-1985-V14, and is not adapted for a precise prediction of the decay heat, in particular for the first minutes after shut-down. It also was not possible to reliably theoretically predict the decay heat, as also discussed in the above report, in particular for the first minutes after shutdown, because rough approximations had to be made and particularly only incorrect descriptions existed of the beta decay of the involved nuclides.
The consequence of the latter is--this is found as result of new data based on a more precise theoretical method of calculating the nuclear .beta. decay of the various fission products, which form the basis of the invention to be described--a considerable overdimensioning of reactor emergency cooling systems up to now.